Golf club heads for "woods" or "drivers" are now commonly made of molded plastic materials with reinforcement fibers or of cast light weight aluminum or other metals. Such clubs have largely superseded natural wood materials such as persimmon or cherry for reasons including stability and availability of materials.
However, the specific gravities of such cast or molded materials are significantly greater than that of persimmon, for example. Therefore, in order to simulate the appearance and weight of a club head made of natural persimmon, the golf club head is provided with a large cavity that opens at the sole. In order to prevent ingress of moisture and for other reasons, the cavity is sealed by filling it with a plastic, such as polyurethane, foamed in suit. The plastic is confined during the foaming and molding process by a sole plate firmly attached to seal the cavity. Typically, the plastic is injected into the cavity via the shaft mounting socket or hosel provided at the neck of the head.
The use of the mounting socket for such purpose often distorts and enlarges about the edge located at the junction of the inner end of hosel and the cavity. The connection between the shaft and the club head is thereby significantly weakened, especially since the inner end of the shaft is not surrounded by a solid, unitary mass as in a conventional club head. Moreover, the lack of a secure connection reduces the sensation of impact transmitted from the club head back to the grip and the golfer's hands.
Apart from the fact that the hole edge may be multilated and enlarged, the surface of the hole is disturbed when hardened plastic is removed preparatory to shaft installation. Such reconditioning of the hole results in poor adhesion between the plastic and the shaft. Moreover, the removal of the hardened plastic remnants is cumbersome and tedious.
One proposed solution, described in Japanese Patent Laid-Open No. 51-106528, is to fix the sole plate temporarily while injecting foaming plastic through a tapped hole and thereafter installing a screw to close the fill hole. This system creates certain problems due to the necessary sequence of steps. Thus the depth of the peripheral step on which the sole plate is mounted varies due to the molding process, for which reason a finishing operation is required. The sole plate must be cut or ground away to achieve continuity between the sole plate and the body of the club head. Grinding creates heat which melts the foamed plastic, redistributing its mass and creating voids. The tapped hole may be fouled, preventing proper attachment of the screw. Control of toe to heel and other weight relationships requires calculated changes in the capacity and configuration of the cavity.